Transforming growth factors (TGF's) are heat and acid-stable peptides which have been isolated from a variety of rodent and human carcinomas and from the conditioned medium (CM) of rodent and human tumor cell lines and from retrovirus transformed cells. TGF's are able to reversibly confer upon normal cells certain properties ascribed to the transformed phenotype, namely a reduced serum requirement in monolayer culture and a loss of anchorage-dependent growth. TGF's have been isolated from the CM of a human mammary carcinoma cell line (MCF-7) and from the CM of several clones derived from this cell line. Comparable TGF's have also been isolated from a transplantable human mammary adenocarcinoma (Clouser), biopsies of human breast tumor and human milk. These TGF's are: 1. able to compete with epidermal growth factor (EGF) for receptor binding; 2. able to induce the anchorage-independent growth of rat fibroblasts and MCF-7 cells in soft agar and 3. potent mitogens for rat fibroblasts and normal mammary epithelial cells. The TGF's present in the MCF-7 CM, the human tumor biopsies and human milk exhibit a pI of 4.0. The TGF activity which is present in human milk has been partially purified. This activity has a molecular weight of approximately 6,000 and is present in milk at a concentration of approximately 25 micrograms/liter. The biological activity associated with this species is inactivated by reduction but is stable to heat and acid treatment. The TGF activity is apparently not human EGF (pI 4.6) since polyclonal antibodies raised against human EGF fail to detect any EGF in the TGF preparations. Moreover, human EGF and the milk-derived TGF elute at different positions with acetonitrile following reverse phase high performance liquid chromatography.